The tunnel magnetoresistive effect is a phenomenon in which the electrical resistance of a ferromagnetic tunnel junction including an insulating material between two ferromagnetic metal electrodes is varied depending on the relative magnetization directions of the two ferromagnetic electrodes. The magnitude of magnetoresistance is expressed by the following equation:Magnetoresistive effect=[(Antiparallel R−Parallel R)/Parallel R]×100 (%)   (1)
Parallel R represents electrical resistance when the magnetization directions of the two ferromagnetic electrodes are parallel to each other; and Antiparallel R represents electrical resistance when the magnetization directions are antiparallel. This phenomenon was discovered in 1995 (T. Miyazaki and N. Tezuka, J. Magn. Mater., Vol. 139 (1995), L231), and applied researches to a magnetic sensor of a pickup head of a hard disk drive, a ferromagnetic random access memory, and so on have been advanced. In order to achieve a magnetic sensor or ferromagnetic random access memory using this effect, a large magnetoresistive effect is required, and it is desired that the magnitude of magnetoresistive effect at a limited bias voltage can be artificially controlled.
However, the tunnel magnetoresistive effect, first, has a problem in that it is limited by spin polarization (P1 and P2 in equation (2)) of the electrode material.
This relationship is expressed by Julliere's equation shown in below:Magnetoresistive effect=[2P1P2/(1−P1P2)]×100 (%)   (2)
Accordingly, in order to obtain a high magnetoresistive effect, a material with a high spin polarization has to be used. On this account, for example, an Fe—Co Alloy is prepared to increase the spin polarization. However, it is considered that even this technique provides a magnetoresistive effect of at most about 70% or less.
In order to solve this problem, use of a semimetal having a spin polarization of 100% in principle has been attempted. However, since such a semimetal is of a compound, its surfaces are unstable. No material suitable for practical use has been discovered.
Furthermore, for use as a magnetic sensor, the magnetic characteristics, as well as the spin polarization, of the material have to be optimized. Unfortunately, such a new material does not generally provide good magnetic characteristics.
Second, in general, the tunnel magnetoresistive effect is monotonously decreases as bias voltage increases. This decrease of magnetoresistive effect with increasing bias voltage is not easily controlled because it is an intrinsic problem resulting from increase of magnon scattering or phonon scattering. Accordingly, an attempt has been made to, for example, dualize a tunnel barrier wall layer so as to prevent the magnetoresistive effect from decreasing with increasing bias voltage.
On the other hand, Moodera et al. attempted to vary the dependence of magnetoresistance on bias voltage by disposing a polycrystalline nonmagnetic interlayer between a ferromagnetic electrode and a barrier layer (Moodera, Phys. Rev. Lett., vol. 83, 1999, pages 3029–3032).
FIG. 1 shows the dependence of junction magnetoresistive effect of a magnetoresistive effect element on the thickness of a Au interlayer at zero bias and 77 K, in which the horizontal axis represents the thickness (nm) of the Au interlayer and the vertical axis represents the magnetoresistive effect. FIG. 2 shows the dependence of magnetoresistive effect of the same element on bias voltage (V), in which the horizontal axis represents bias voltage (V) and the vertical axis represents the magnetoresistive effect.
However, in the foregoing magnetoresistive element in which the polycrystalline, nonmagnetic interlayer is disposed between the ferromagnetic electrode and the barrier layer, the scattering magnetoresistive effect of electrons is rapidly reduced as the thickness of the nonmagnetic interlayer increases, because the nonmagnetic interlayer is formed of a polycrystalline material. Although the bias voltage dependence of the magnetoresistive effect has been successfully controlled to some extent, characteristics sufficient for practical use have not been achieved.
Accordingly, it is desired to develop an element in which electron scattering of a nonmagnetic interlayer is reduced, the element whose magnetoresistive effect does not seriously reduced even if the thickness of the interlayer is large, and the element from which oscillation of spin polarization in a nonmagnetic layer can be drawn out as magnetoresistive effect.
Third, it has been pointed out that a barrier layer formed of MgO (001) single crystal produces a giant tunnel magnetoresistive effect (J. Mathon, et al., Physical Review B, volume 63 (2001), pages 220403(R)-1-4). Also, a giant tunnel magnetoresistive effect of more than 60% has been experimentally obtained (M. Bowen et al., Applied Physics Letters, volume 79, number 11 (2001), pages 1655–1657). In this instance, a single-crystal substrate is necessary to form an element. This however makes it difficult to form the element on a silicon LSI, and thus a challenge for realizing a magnetic random access memory has not yet overcome.